1. Field of the Invention
The present Invention relates to an apparatus for monitoring ventilation or integrated circuits in an electronic apparatus.
2. Description of the Related Art
In order to protect integrated circuits in an electronic apparatus against thermal damage, it is the common practice to ventilate, by a fan, a space within an electronic apparatus where integrated circuits are disposed. To monitor the ventilated state of the space, it is known to measure, by a is temperature sensor, temperature at an appropriate point within the space. The measure of temperature is compared with a predetermined upper limit in temperature. A command is issued to suspend operation of the electronic apparatus when the measure of temperature reaches the predetermined upper limit. This art is described as conventional in JP-A 4-295223. According to JP-A 4-295223, this conventional art involves a potential problem that no alarm is produced before the electronic apparatus stops its operation and thus there is no time to store data beforehand.
To solve this problem, JP-A 4-295223 teaches comparing the measure of temperature with a warning temperature value that is lower than the predetermined upper limit and producing an alarm when the measure reaches the warning temperature value. It also teaches measuring speed of a ventilation fan and airflow rate or speed upstream of the fan with respect to flow of air passing through the space. The measures of the fan speed and the airflow rate or speed are processed in a predetermined logic to produce two different pieces of information regarding causes of the temperature rise in the space. According to the logic, when the measure of fan speed drops below a predetermined value, an alarm is produced that the fan is in trouble, and when the measure of airflow rate or speed drops below a predetermined value, another alarm is produced that an air filter is plugged.
U.S. Pat. No. 5,477,417 teaches the use of an integrated circuit (IC) device including a built-in positive temperature coefficient (PTC) thermistor. The thermistor is disposed near an integrated circuit chip that releases heat in operation. The thermistor exhibits a characteristic that its resistance changes in response to a change in temperature. The IC device includes an abnormal temperature signal output chip. This chip includes an inverter gate and a fixed resistor that is connected between a terminal on which a source of voltage is impressed and one end of the thermistor. The other end of the thermistor is connected with a terminal that is grounded. An input end of the inverter gate is connected with an intermediate point between the thermistor and resistor. The other or output end of the inverter gate is connected with a terminal that is used as an output terminal. When the temperature to which the thermistor is exposed becomes a preset temperature value, a potential on the output terminal changes from its high level (ON) to its low level (OFF). This U.S. patent also teaches an electronic apparatus that has mounted in a frame thereof a number of printed circuit boards. A large number of IC devices are mounted on each of the printed circuit boards. Among them, some are IC devices with a built-in thermistor. Fans are mounted to a rear panel attached to the frame and an air inlet duct is formed in a front panel attached to the frame. A plurality of airflow paths is defined within the frame between the air inlet duct and the fans. A controller and an alarm unit are arranged in the frame. The controller is programmed to cause the alarm unit to produce alarm signal and a fan driver to increase speed of the fans when it determines that potential at one or more of the thermistor built-in type IC devices indicates abnormally high temperature. It is proposed to effect reduction in operation clock frequency of a CPU when it is determined that the temperature has reached abnormally high value.
Let us now consider an IC ventilation monitoring apparatus where a temperature sensor is so disposed as to detect temperature on downstream side, with respect to flow of ventilated air, within a casing and an integrated circuit within the casing includes a static random access memory (SRAM). SRAM generates small quantity of heat as compared to quantity of heat generated by the whole system. But, it has small package size and heat capacity, increasing possibility that temperature of SRAM becomes excessively high. Thus, the above-mentioned monitoring apparatus cannot detect the abnormal event of SRAM.
Let us next consider another IC ventilation monitoring apparatus where temperature sensors, in the form of IC, are mounted on an integrated circuit device. In this case, in order to protect the integrated circuit device against thermal damage, highly accurate anticipation or information is required to determine the number and mount sites of temperature sensors and limit temperature values.
If a temperature sensor is disposed on the discharge side of an integrated circuit device, what is monitored is temperature of the overall stem. Thus, a limit temperature value is set after considering an allowable increase of temperature of the overall system. The ambient temperature and the total load on the system determine the increase of internal temperature of the system. Thus, a relatively high temperature value is set, as the limit temperature value, after considering a great contribution made by a CPU, which generates a large quantity of heat, to an increase in the system internal temperature.
Suppose that a CPU and a SRAM are mounted to different cards, respectively, and the path for inflow of cooling air to the SRAM mounted card is plugged, causing an abnormal event that the internal temperature of this card becomes excessively high to induce thermal damage. This abnormal event cannot be detected if the relatively high temperature value is set as the reference temperature of the temperature sensor.
This problem remains unsolved ever if a temperature sensor is mounted adjacent an integrated circuit, such as a CPU, with a large heat generating capability so as to monitor its temperature. This arrangement of the temperature sensor poses another problem that since it is less tall than a CPU with heat dissipating fins, the temperature sensor cannot monitor temperature of air having past the CPU.
The temperature of air having past a CPU varies with its ambient temperature and load, making it very difficult to set a limit temperature value, with which a measure of temperature is compared. The temperature of air having past around the CPU alters considerably in response to a change in mode in which the CPU operates. Suppose that the flow rate of air is considerably lower than a level expected and the load on the CPU is low so that the temperature of air having past around the CPU stays lower than the limit temperature value. This ventilation environment may allow an increase in internal temperature of the adjacent SRAM to a level causing its thermal damage, leaving this abnormal event occurring in the SRAM undetected.
Protecting all of the integrated circuits requires a number of temperature sensors to be mounted on a card. However, the other integrated circuits on the card make it difficult for such temperature sensors to be disposed at their appropriate positions.
Let us now consider what must be accounted in protecting integrated circuits within an electronic apparatus if counting the number of pulses generated by a speed sensor of a fan and/or determining whether the fan locks are used in producing an alarm.
In the case where integrated circuits are highly condensed within an electronic apparatus to such a degree as to induce a great pressure loss, a plurality of fans are arranged in series with respect to the flow of air passing around the integrated circuits. According to this arrangement, even if it fails to operate, a fan continues to rotate as its blades are subject to static pressure due to the other fans.
There are cases where it is difficult to set a limit rotational speed value of the fans depending on the site at which the electronic apparatus is mounted and the layout of the integrated circus to be protected.
The fan speed is expected to have an authority over temperature where the integrated circuits are disposed. However, the relation between the fan speed and the temperature is difficult to hold in a system using a plurality of fans, making it very difficult to determine the fan speed limit value.
An object of the present invention is to provide an IC ventilation monitoring apparatus that does not require complicated work for setting a limit temperature value with which a measure of temperature is compared.